Automatic transmission fluid is a composition based upon selected grades of petroleum hydrocarbon oils incorporating essential performance additives. Additives include, for instance, anticorrosion agents, antifoaming agents, viscosity improvers and a red dye. The latter is added to provide an immediately visible characteristic to distinguish the automatic transmission fluid from other oily fluids used in automotive systems including, for example, lubes, brake and power steering fluids. The nature and degree to which the additives are present in automatic transmission fluid is specified by the automotive manufacturers notably the Dextron.RTM. series of specification issued by General Motors Corporation and the Mercon.RTM. specifications issued by the Ford Motor Company, specifications which have also been widely adopted by other automotive manufacturers. In all of these specifications, where a red dye is called for, the dye used is either the dry powder dye identified generically in the "Colour Index," a joint publication of the American Association of Textile Chemists and Colorists and the Society of Dyers and Colourists (UK), as C.I. Solvent Red 24 or its tinctorially equivalent analogue C.I. Solvent Red 164. The disclosure of this publication is incorporated herein by reference. The latter dye is supplied as a liquid dye concentrate and is now generally preferred over C.I. Solvent Red 24 because of its greater convenience of handling. Both C.I. Solvent Red 24 and 164 belong to the chemical class of azo dyes.
After the conventional passenger automobile fitted with an automatic transmission has been driven for about 40,000 miles, the transmission fluid becomes degraded chemically. During this period the dye additive also degrades and the originally bright pink or red transmission fluid turns initially to an orange shade, and ultimately to a dark brown color. This loss of red color is then often used as a visual indication that the fluid needs to be replaced which incurs expense and possible environmental hazards if the spent fluid is not disposed of properly. Prolonged use of degraded automatic transmission fluid, beyond its manufacturers recommended lifespan, considerably accelerates wear and damage to the automotive transmission system.
More recently, the manufacturers of ATF have made products with a considerably extended usable lifespan, i.e., 100,000 miles or even 200,000 miles or more in standard passenger vehicles. When these improved fluids are colored with C.I. Solvent Red 24 or C.I. Solvent Red 164, however, the dyes continue to degrade at essentially the same rate in the more stabile fluids as they did in the older grades of ATF. This may cause some to believe that the extended life characteristics of the ATF have been misrepresented because, in the automotive servicing industry, "everybody knows" that when ATF changes from red to brownish-orange, it is life-expired, notwithstanding the manufacturers' claims to the contrary. Consequently, the use of a red dye with a persistence commensurate to the extended life of the ATF has become very desirable.